1. Field of the Invention
This invention relates to a dielectric ceramic composition which is produced by using barium titanate as a base and is suitable for use in capacitors, and a process for producing the same.
2. Description of the Prior Art
Barium titanate and compositions prepared by mixing barium titanate with shifters and depressers have heretofore been sintered at a high temperature of 1,300.degree. to 1,400.degree. C. and used in capacitors. However, when they are sintered at such a high temperature, there are caused wear of a setter made of expensive zirconia or the like and a sintering furnace, and moreover a large amount of energy is necessary for the sintering. Therefore, the cost of the resulting capacitor is high. Further, in order to produce a multilayer ceramic capacitor by using the conventional composition, an expensive noble metal such as platinum or palladium which can withstand a high sintering temperature should be used as a material for inner electrodes, and the cost of the resulting multilayer ceramic capacitor is very high. Accordingly, there is ardently desired a dielectric ceramic composition capable of being sintered at a low temperature which permit reduction of the wear of a setter and a sintering furnace, and also employment of inner electrodes comprising inexpensive silver as the main constituent for producing a multilayer ceramic capacitor.
On the other hand, for practical use in a capacitor, there is needed, as a composition having a high relative permittivity, a dielectric ceramic composition satisfactory in X7R characteristic, Z5 characteristic and Y5V characteristic of EIA standard and B characteristic and F characteristic of JIS standard. As to X7R characteristic and B characteristic, the composition should have a relatively high relative permittivity, and the temperature dependence of the relative permittivity should be small. As to Z5V characteristic, YSV characteristic and F characteristic, the temperature dependence of the relative permittivity may be large but the relative permittivity should be very high.
In order to obtain a multilayer ceramic capacitor of small size and large capacity according to either of the above-mentioned standards, the relative permittivity of dielectric ceramic composition should be high and the thickness of dielectric layer between internal electrodes should be thin. When the grain size of sintered body is large and not uniform, the breakdown voltages is lowered, so that problems are caused in practice. Therefore, there is particularly ardently desired a dielectric ceramic composition which has a high relative permittivity, a uniform and very small grain size, and can be sintered at a low temperature.
Systems containing bismuth oxide have been put to practical use as a composition capable of being sintered at a low temperature, but are disadvantageous, for example, in that they have a low relative permittivity, that bismuth is volatilized during sintering, resulting in unstable production, and that the dielectric loss is large in high-frequency regions. Therefore, they are eagerly desired to be improved.
On the other hand, copper oxide is known as one of sintering aids which are free from the disadvantages of the above-mentioned bismuth systems and can lower the sintering temperature of barium titanate. It is shown in Trans. Brit. Ceram. Soc., 74, 165, (1975) that a sintered body having a substantially theoretical density can be obtained at a sintering temperature of 1,200.degree. C. by adding cupric oxide to barium titanate in an amount of 0.5 mole % less based on the total amount.
U.S. Pat. No. 4,244,830 discloses a production method which comprises adding a compound capable of forming a CuO--Cu.sub.2 O eutectic mixture or a CuO--Cu.sub.2 O.Me.sup.IV O.sub.2 eutectic mixture (wherein Me.sup.IV O.sub.2 is an oxide of at least one element selected from the elements in group IV and is not incorporated into a perowskite lattice) to perowskite-forming compounds, and sintering the resulting mixture in the temperature range of 1,000.degree. to 1,250.degree. C.
However, dielectric ceramics obtained by these methods have large and unever grain sizes, and in some cases, giant grains having a size of as large as several tens microns are produced. Therefore, the dielectric ceramics have unstable characteristics and are substantially unusable particularly in multilayer ceramic capacitors.
On the other hand, U.S. Pat. No. 4,222,885 discloses a method for producing dielectirc ceramics which comprises adding an oxide capable of forming a CuO--MeOx (wherein MeOx is an oxide of at least one element selected from the elements in III, V, VI and VII groups of the periodic table and is not incorporated into a perowskite lattice) eutectic mixture to perowskite-forming compounds, and sintering the resulting mixture in the temperature range of 1,000.degree. to 1,250.degree. C. According to this method, a sintered body having relatively small and uniform grains can also be obtained, but the dielectric loss (tan .delta.) is large, so that the method involves problems in practice. Thallium gives the dielectric ceramic having the best characteristics, but it is highly toxic and expensive, which becomes a serious problem in practice.
As another sintering aid capable of lowering the sintering temperature of barium titanate, lithium fluoride is known. U.S. Pat. No. 4,082,906 discloses a composition containing 90 to 99.75% by weight of barium titanate and 0.25 to 10% by weight of lithium fluoride. Japanese Patent Application Kokai (Laid-Open) No. 160963/82 discloses a composition containing 1.5 to 10% by weight of lithium fluoride. Further, Japanese Patent Application Kokai (Laid-Open) No. 135178/83 discloses a method using a combination of 0.5 to 2.5% by weight of lithium oxide and 1.5 to 8% by weight of zinc fluoride. Japanese Patent Application Kokai (Laid-Open) No. 20781/83 discloses a method using a combination of 0.5 to 2.5% by weight of lithium oxide and 1.5 to 8% by weight of cupric fluoride.
However, addition of fluoride of lithium, a univalent metal in a large amount of 0.25 to 10% by weight is disadvantageous in that the durabilities such as high-temperature load characteristic (life test), moisture resistance characteristic and the like are liable to be deteriorated. Further, since fluorides such as lithium fluoride and the like are highly volatile compounds, their employment is disadvantageous in that when a composition containing them in a large amount is sintered, deformation of a sintered body is apt to be caused, so that a stable product is difficult to obtain.
In addition, a system containing lithium fluoride alone as an additive brings about good results only when the molar ratio of TiO.sub.2 to BaO.sub.2 in barium titanate is 0.97 to 0.98. In the case of ordinary barium titanate in which the molar ratio is about 1, the insulation resistance and the relative permittivity are low, so that no good result can be obtained. And lithium fluoride systems comprising the combination of lithium oxide and zinc fluoride or cupric fluoride also has the same disadvantages as described above.
That is to say, in prior art, there is strongly desired an advent of a highly reliable dielectric ceramic composition which has high relative permittivity, a high insulation resistance, a small dielectric loss and fine and uniform grains, can be sintered at a low temperature, and permits stable production of a sintered body.